Biometric technologies enable the authentication of users based on an individual's physiological, cognitive or behavioral characteristics. Performing authentication based on the combination of information available across these traits further improves robustness. For example, conversational biometrics measures both the voice and the knowledge response from the users to perform improved authentication.
Considered in isolation, many biometrics technologies assume relatively cooperative users. Cooperative users obey the basic rules set out for the use of the biometrics system. In reality, however, imposters will attempt to be authenticated by the system by whatever means possible. For example, in an automatic speaker verification system impostors could play back audio material recorded from target speakers in order to deceive the system. More sophisticated approaches may also include speech synthesis for the speaker of interest.